Electrically powered exit signs, traffic signals, task lights and other devices are widely used. Fluorescent lamps are used to provide illumination in typical electrical devices for general lighting purposes because they are more efficient than incandescent bulbs in producing light. A fluorescent lamp is a low pressure gas discharge source, in which light is produced predominantly by fluorescent powders activated by ultraviolet energy generated by a mercury plasma forming an arc. The lamp, usually in the form of a tubular bulb with an electrode sealed into each end, contains mercury vapor at low pressure with a small amount of inert gas for starting. The inner walls of the bulb are coated with fluorescent powders commonly called phosphors. When the proper voltage is applied, the plasma forming an arc is produced by current flowing between the electrodes through the mercury vapor. This discharge generates some visible radiation. The ultraviolet in turn excites the phosphors to emit light.
Two electrodes are hermetically sealed into the bulb, one at each end. These electrodes are designed for operating as either “cold” or “hot” cathodes or electrodes, more correctly called glow or arc modes of discharge operation. Electrodes for glow or cold cathode operation may consist of closed-end metal cylinders, generally coated on the inside with an emissive material. Conventional cold cathode lamps operate at a current on the order of a few hundred milliamperes, with a high cathode fall or voltage drop, something in excess of 50 volts. CCFLs are not appreciably affected by starting frequency because of the type of electrode used CCFLs emit light in the same way as to standard hot electrode lamps. The latter type operate as normal glow discharges and their electrodes are uncoated hollow cylinders of nickel or iron. The cathode fall is high and to obtain high efficacy or power for general lighting purposes, conventional lamps are made fairly long, about 2–8 feet, with a diameter of about 25–40 millimeters. About 2000 volts is required for starting these conventional lamps and about 900 to 1000 volts for running.
The advantages of CCFLs, compared with the hot electrode fluorescent lamps are that they have a very long life (usually) 15000 hours or more) in consequence of their rugged electrodes, lack of filament and low current consumption. They start immediately, even under cold ambient conditions. Their life is unaffected by the number of starts. Also, they may be dimmed to very low levels of light output.
U.S. Pat. No. 5,440,467 issued Aug. 8, 1995 to Lautzenheiser teaches a task light. The light assembly is provided for illuminating a work surface below and in front of the light assembly, and includes a housing configured for mounting over the work surface with an elongated linear light source supported in its housing. A tubular lens is built into and part of the housing, and includes prism-shaped triangular rings on its inside surface for controlling the light from the light source onto the work surface therebelow.
Other electric light sources currently include incandescent, compact and tubular fluorescent lamps, plasma arc lamps, neon lamps, electroluminescent (EL) lamps and light emitting diodes (LEDs). The fluorescent, plasma and neon lamps all require a voltage differential which may be achieved with electrodes in the interior of the tube or with conductive coatings or elements on the tube exterior that are adapted to be coupled to a source of electrical energy. High frequency energy fields, such as are generated by tesla coils or the like, can also cause a radiating ionization of the gases without the need for a direct connection to a source of electrical energy.